Various types of floating chuck mechanisms have been proposed. For example, in the cases where a workpiece is supported and held between a pair of center members of a processing machine such as a cylindrical grinding machine so that the workpiece is driven to rotate, an eccentricity (i.e., misalignment) between a center of a workpiece holding portion (i.e., clamp jaws that press and sandwich the workpiece from a direction perpendicular to a rotational axis of the workpiece) and a rotational axis of both the center members (that sandwich the workpiece from a rotational axis direction thereof) is corrected by means of the clamp jaws (i.e., clamp jaw portion) serving as center misalignment correcting means. For example, JP2003-145326A discloses a floating chuck mechanism in which clamp jaws (i.e., clamp jaw portion) are attached to a diaphragm that can float in a radial direction thereof. The clamp jaws automatically perform a centripetal movement because of a return deformation of the diaphragm. In addition, JP11-254214A discloses a floating chuck in which each clamp jaw (i.e., clamp jaw portion) is driven by a hydraulic cylinder so that force for holding a workpiece is prevented from decreasing by means of centrifugal force even at a time of high speed rotation of the workpiece. Further, JP5-93710U discloses a floating chuck in which clamp jaws (i.e., clamp jaw portion) perform a centripetal movement by means of a swing arm and a spring.
According to the floating chuck mechanism disclosed in JP2003-145326A, the clamp jaw portion performs the automatic centripetal movement. A diameter of the workpiece that can be supported or held by the clamp jaw portion is determined on the basis of a movable area of the diaphragm. Thus, the chuck (that is equipped with the clamp jaw portion) used for processing a workpiece having a large diameter cannot be used for processing a workpiece having a small diameter and in such a case, the chuck should be replaced. In addition, according to the floating chuck disclosed in JP11-254214A, the clamp jaw portion also automatically performs the centripetal movement. The clamp jaws are driven by the hydraulic cylinder mechanism so as to meet a wide range of a target diameter of a workpiece, however, such structure is complicated. Further, according to the floating chuck disclosed in JP5-93710U, the clamp jaw portion automatically performs the centripetal movement by means of the swing arm. However, in this case, the clamp jaws only support a workpiece having a limited range diameter. Thus, the chuck (that is equipped with the clamp jaw portion) used for processing a workpiece having a large diameter cannot be used for processing a workpiece having a small diameter and in such a case, the chuck should be replaced. Furthermore, according to structures of the floating chuck mechanisms disclosed in JP2003-145326A and JP5-93710U, respectively, the force for holding a workpiece by the clamp jaw portion is uneven, which may lead to a distortion of the workpiece.
Thus, a need exists for a floating chuck mechanism in which a clamp jaw portion can meet a wide range diameter of a workpiece and which has a simple structure.